Double-jet precipitation is a common practice in the making of silver halide emulsions. Silver salt solution and halide salt solution are introduced simultaneously, but separately, into a precipitation reactor under mixing. In order to achieve desired crystal characteristics, typically, the silver ion activity or the halide ion activity is controlled during the precipitation by adjusting the feed rates of the salt solutions using either a silver ion sensor or a halide ion sensor.
Formation of silver halide emulsions typically involves a crystal nuclei-forming step wherein addition of silver ion results primarily in the precipitation of new crystal nuclei, and a subsequent double-jet growth step wherein the rate at which silver and halide are introduced is controlled to primarily grow the crystals already previously formed while avoiding the formation of new seed grains, i.e., renucleation. Addition rate control to avoid renucleation, and thereby generally provide for a more monodisperse grain size final grain population, is generally well known in the art, as illustrated by Wilgus German OLS No. 2,107,118; Irie U.S. Pat. No. 3,650,757; Kurz U.S. Pat. No. 3,672,900; Saito U.S. Pat. No. 4,242,445; Teitschied et al European Patent Application 80102242; “Growth Mechanism of AgBr Crystals in Gelatin Solution”, Photographic Science and Engineering, Vol. 21, No. 1, January/February 1977, p. 14, et seq. The term “critical crystal growth rate” is used in the art to describe the growth rate obtained at the maximum rate of silver ion and halide ion addition which does not produce renucleation. While maintaining silver and halide addition rates below that which form new grain populations is advantageous during grain growth in terms of controlling the emulsion grain population characteristics, it also can restrict obtainable emulsion concentrations (i.e., batch yields) and lengthen emulsion manufacturing times.
U.S. Pat. Nos. 5,549,879; 6,043,019; 6,048,683 and 6,265,145 disclose double jet techniques for preparing silver halide grains wherein silver and halide salt solutions are added at a “pulsed flow” rate designed to generate a second grain population (i.e., at a rate above that which would provide for the critical crystal growth rate), with multiple short “pulses” being separated by hold periods designed to allow the new grain population to be ripened out. U.S. Pat. No. 5,549,879, e.g., discloses introducing an aqueous silver nitrate solution from a remote source by a conduit which terminates close to an adjacent inlet zone of a mixing device, which is disclosed in greater detail in Research Disclosure, Vol. 382, February 1996, Item 38213. Simultaneously with the introduction of the aqueous silver nitrate solution and in an opposing direction, aqueous halide solution is introduced from a remote source by a conduit which terminates close to an adjacent inlet zone of the mixing device. The mixing device is vertically disposed in a reaction vessel and attached to the end of a shaft, driven at high speed by any suitable means. The lower end of the rotating mixing device is spaced up from the bottom of the vessel, but beneath the surface of the aqueous silver halide emulsion contained within the vessel. Baffles, sufficient in number to inhibit horizontal rotation of the contents of the vessel are located around the mixing device. The described apparatus is operated in a “pulse flow” manner comprising the steps of: (a) providing an aqueous solution containing silver halide particles having a first grain size; (b) continuously mixing the aqueous solution containing silver halide particles; (c) simultaneously introducing a soluble silver salt solution and a soluble halide salt solution into a reaction vessel of high velocity turbulent flow confined within the aqueous solution for a time t, wherein high is at least 1000 rpm; (d) simultaneously halting the introduction of the soluble silver salt solution and the soluble halide salt solution into the reaction for a time T wherein T>t, thereby allowing the silver halide particles to grow; and (e) repeating steps (c) and (d) until the silver halide particles attain a second grain size greater than the first grain size. Advantages of the pulse flow technique described include permitting easier scalability of the precipitation method. There is no disclosure of use of such pulse flow technique to enable larger emulsion concentrations (i.e., batch yields) or shorten emulsion manufacturing times. To the contrary, the disclosed need for relatively long hold times between pulsed addition of silver and halide salts can result in longer manufacturing times.
U.S. Pat. No. 6,043,019 teaches the use of pulsed flow growth for high bromide tabular grain emulsion after a speed-enhancing amount of iodide is added to the reaction vessel. Such emulsions are more robust for chemical sensitization, have an improved speed-granularity relationship and they exhibit reduced intrinsic fog. Thus, pulsed growth appears to affect iodide incorporation in tabular grains in a beneficial way. There is no disclosure of use of such pulse flow technique to enable preparation of high bromide emulsion grains having desired performance characteristics while increasing emulsion concentrations (i.e., batch yields) or shorten emulsion manufacturing times. To the contrary, the pulsed addition of silver halide salts is described specifically for only the outer 5 to 50 percent (and more preferably for only the outer 5 to 30 percent) of silver forming the final tabular grain emulsion, and the pulses are separated by hold times. Further, there is no disclosure of use of the described process to prepare high bromide cubical emulsion grains.
U.S. Pat. No. 6,048,683 teaches a pulse flow process for the preparation of high chloride cubical silver halide grains grown in the presence of a thioether ripening agent wherein the resulting silver chloride grains exhibit an average grain roundness coefficient, n, in the range of from 2 to less than 15. U.S. Pat. No. 6,265,145 teaches a process for the preparation of high chloride cubical silver halide grains containing from 0.05 to 3 mole percent iodide where iodide is incorporated in the grains by introducing at least a silver salt solution into the dispersing medium at a rate such that the normalizing molar addition rate Rn is above 5×10−2 min−1 where Rn satisfies the formula       R    n    =                    Q        f            ⁢              C        f              M  where Qf is the volumetric rate of addition, in liters/min, of silver salt solution to the reaction vessel, Cf is the concentration, in moles/liter, of the silver salt solution, and M is the total moles of silver halide in the host grains in the reaction vessel at the precise moment of addition of the silver salt solution. There is no disclosure, however, of use of the above processes to prepare high bromide silver halide cubical grain emulsions.
U.S. 2004/0018456 discloses that normalized shell molar addition rates substantially higher than critical crystal growth rates typically determined in accordance with prior art techniques may be employed for preparation of monodisperse high bromide cubic emulsions. While reagent addition rates only slightly greater than that which would be associated with such conventionally determined critical crystal growth rates are believed to simultaneously result in both renucleation and growth of the pre-existing grain cores as well as the renucleated seeds, and thus a decrease in grain size uniformity (i.e., increase in polydispersity), it is disclosed that where the normalized shell molar addition rate is further increased to higher levels (i.e., where Rs, is above 1.0×10−3 min 2, Rs satisfying the formula:       R    s    =            M      s                      M        t            ⁢              t        s        2            where Ms is the number of moles of silver halides added to the reaction vessel during the formation of the shell, ts is the run time, in minutes, of the silver salt solution for the formation of the shell, and Mt is total moles of silver halide in the reaction vessel at the end of the precipitation of the shell), substantially all of the added reagent is precipitated into fine grains which then ripen primarily only onto the larger pre-existing host grain cores, resulting in a relatively monodisperse emulsion.
While substantially all of the added reagent is precipitated into fine grains which then ripen primarily only onto the larger pre-existing host grain cores in accordance with the process described in U.S. 2004/0018456, it has been found that depending upon other process conditions, there still may exist maximum addition rates above which the fine grains formed via high normalized shell molar addition rates become stable and result in the formation of a minor, though still generally undesirable, fraction of a secondary grain population. The stabilization of these fine grains is a result of the inability of the system to effectively ripen all of the precipitated fine grains onto the grain cores of the primary grain population during shell growth. It would be desirable to provide a process that extends the conditions under which high bromide cubical grain emulsions of a desired grain size may be obtained under high normalized shell molar addition rates while minimizing the occurrence of secondary grain populations.